Quartz crystal oscillator



Feb. 22; 1944.

e. T. ROYDEN QUARTZ CRYSTAL OSCILLATOR Filed May 4;"1942 I I'EEQUE/VC YINVENTOR 6. TRog'ae/I ATTORNEY Patented Feb. 22, 1944 QUARTZ CRYSTALOSCILLATOR George T. Royden, South Orange, N..J.,

assignor Federal Telephone and Radio Corporation,

a corporation of Delaware Application May 4, 1942, Serial Ne. 441,585

16. Claims.

This invention relates to electron tube oscillator generators and moreespecially to such generators provided with a crystal type frequencystabilizer.

A principal object of the invention relates to a crystal stabilizedoscillator generator wherein the stabilization effect of the crystal issubstantially equalized for both positiverand negative departures of theoscillator from a standard frequency.

Another principal object is to provide a vacuum tube oscillatorgenerator which is to be stabilized by a quartz crystal and wherein aduplex comparison network is provided for comparing the resonantfrequency of the crystal with the generated frequency of the oscillator,one of said networks being effective for a positive departure and theother being effective for a negative departure of the generator from astandard frequency.

Another object is to provide the frequencydetermining circuit of anelectron tube oscillator generator with a pair of grid-controlled tubeswhich are selectively effective on the frequencydetermining circuitunder control of, and in accordance with, the direction of departure ofthe generated oscillations from the resonant frequency of the crystal.

A feature of the invention relates to an electron-tube oscillatorgenerator whose tank circuit feeds a triple branched frequencystabilizing network, one branch comprising mainly inductive reactance,another branch comprising mainly capacity reactance, and the thirdbranch comprising mainly the reactance of the crystal, said branchesbeing connected to an electron tube network which produces a pair ofindependent electron streams selectively effective to control thefrequency of the generated oscillations in accordance with the phaserelation of the voltage in said crystal branch compared with thevoltages in the said other two branches.

Another feature relates to an electron-tube oscillator generator havinga tank circuit shunted in part by a variable impedance which controlsthe generated frequency, the value of said impedance being determined bya pair of independent electron discharge paths which are in turnselectively effective in accordance with the amount and phase ofdeparture of the generated frequency from the resonant frequency of thecrystal.

Another feature relates to an oscillator generator and a crystalfrequency stabilizer therefor, including a pair of multi-grid electrontubes having their control grids excited in phase with a voltage derivedfrom the tank circuit of the oscillator and in phase with the excitationof the crystal, each of said tubes having another impedance-controllinggrid the relative phases of excitation of which are determined by thedeparture of the generated frequency from the resonant frequency of thecrystal.

A further feature relates to a crystal stabilized network for electrontube oscillators comprising a balanced triple-branched network, onebranch comprising mainly a capacity reactance, the second branchcomprising mainly an inductive reactance and the third branch comprisingthe reactance of the crystal, each of said branches including .abalancing resistance whereby the voltage developed across the capacityand inductance 'reactances aresubstantially equal when the generatedfrequency is substantiallythe same .as the crystal resonant frequency,said voltages determining respectively the impedance of a correspondingpair of grid-controlled electron tubes which in turn selectivelydetermine the automatic tuning of the tank circuit of the generator.

A further feature relates to an electron tube oscillator generatorhaving a tank circuit with a variable-impedance electron tube as atuning control element, and a double rectifier arrangement connected tothe control grid of said tube for determining the conductivity thereof,the conductivity of the sections of said rectifier being selectivelycontrolled by a three-branch network having a capacity arm,an.inductance arm and a crystal reactance arm.

A still further feature relates to the novel interconnection of partswhich cooperate to produce a highly efiicient and accurate frequencystabilizer for electron tube generators.

Other features and advantages not particularly enumerated will revealthemselves upon consideration of the ensuing description and claims.

While the invention will. be disclosed in connection with certainspecific forms of tubes and related apparatus, this is done merely toexplain the inventive concept and not by way of limitation. In thedrawing,

Fig. l is a schematic circuit diagram of one embodiment of theinvention.

Fig. 2 is a modified embodiment of the invention..

Figs. 3 and 4 are respectively an explanatory curve and vector diagramrelating to the embodiment of Fig. 2.

Referring to Fig. l, the thermionic vacuum tubes I, 2, 3 and 4 arefurnished heater current from battery 5 and plate and screen currentfrom battery 6. The oscillator tube I has a resistor and capacitor 8 inits cathode circuit and resistor 9 and capacitor Ill in its screencircuit. Plate current is obtained through choke coil The oscillatortank circuit contains an inductor l2 and a capacitor I3; These areconnected to the grid, cathode (through capacitor 8) and anode (throughblocking capacitor l4) of the oscillator tube l as shown in Fig. 1.

A portion of the oscillator tank circuit is shunted (in effect) byinductor l5 and vacuum tube 2, the effective amount of reactancecontributed to the oscillator tank circuit being controlled by the gridand screen voltages of vacuum tube 2. These voltages are in turncontrolled by the currents passing through vacuum tubes 4 and 3respectively.

. 'The radio frequency potential existing in the oscillator tank circuit(at the end connected to the-anode) is impressed through resistors l6,I1 and I8 on an inductor I9. a quartz crystal 2D and a capacitor 2|respectively. The potential drop across the quartz crystal 20 isimpressed on the grids of vacuum tubes 3 and 4. This potential will besmall and in phase with the oscillator tank circuit voltage if thefrequency of the oscillator is equal to the resonance frequency of thequartz crystal. The resistance of resistor I1 is chosen so; that it islarge compared with the equivalent series resistance of the quartzcrystal at resonance. The voltage across the quartz crystal will lead inphase the voltage from the oscillator tank circuit for frequencies abovethe resonance frequency of the quartz crystal and lag in phase forfrequencies below resonance. The magnitudes of resistor l6 and inductorl9 are such that the voltage across inductor I9 is substantially leadingin phase with respect to the oscillator tank circuit voltage. Capacitor22 serves to prevent screen current passing through inductor 9 thuspreserving balance with respect to vacuum tube 4. The magnitudes ofresistor I8 and capacitor 2| are such that the voltage across capacitor2| is substantially lagging in phase with respect to the oscillator tankcircuit voltage. The magnitudes of l6, l8, l9 and 2| are such that thevoltage across capacitor 2| is substantially the same as that acrossinductor l9. Furthermore, resistors .23 and 24 are substantially equalalso resistors 25 and 26 and capacitors 21 and 28 in order to preservebalance between the circuits containing vacuum tubes 3 and 4. preventrapid changes .in plate potential of vacuum tubes 3 and 4. 4

If the frequency of the vacuum tube oscillator is above the resonantfrequency of the quartz crystal 20, the grid voltage of vacuum tube 3will be leading (with respect to the oscillator tank circuit) andsubstantially in phase with the screen voltage. Since these potentialsare normally near the cut-off point of the vacuum tube characteristic(point at which plate current drops to zero), only the positive portionof the radio frequency cycle permits plate current to flow. An increaseof plate current in vacuum tube 3 results in a greater voltage drop inresistor 23 and consequently a drop in screen voltage of the reactancecontrol vacuum tube 2. This decreases the effectiveness or coupling ofthe inductor Hi to the oscillator tank circuit thereby lowering thefrequency of oscillation. r

If the frequency of oscillation of the vacuum Capacitors 29 and 3|! tubeoscillator is below the frequency of resonance of the quartz crystal 20,the grid voltage of vacuum tube 4 will be lagging (with respect to theoscillator tank circuit) and substantially in phase with the screenvoltage. Vacuum tube 4 will then pass current which raises the potentialapplied to the grid of the reactance control tube 2. Such a change inimpedance of vacuum tube 2 increases the coupling of inductor |5 to thevacuum tube oscillator tank circuit and raises its frequency ofoscillation.

Thus it will be seen that frequency of oscillation will automatically becorrected until it coincides with the frequency of resonance of thequartz crystal.

Referring to Fig. 2, there is shown a simplified embodiment of theinvention wherein the vacuum tubes 3 I, 32 and 33 are furnished filamentcurrent from battery 34. Plate current is supplied to vacuum tubes 3|and 32 by battery 35 through a radio frequency choke coil 36. Anoscillating tank circuit comprising inductor'3! and capacitor 38 isconnected to the cathode (via a tap on the coil 31) to the grid and tothe anode, via blocking capacitor 39, of vacuum tube 3|.

Energy from-the oscillating tank circuit is applied through resistors40, 4| and 42 to capacitor 43, quartz crystal 44 and inductor 45respectively, returning to ground and the oscillating tank circuit.Crystal 44 connects directly with ground, while capacitor 43 andinductor 45 have their connection to ground through capacitor 46 whichis of such capacitance that the voltage drop across it for the radiofrequency currents that flow is negligible. i

The voltage drop across capacitor 43 is applied to one plate of the dualrectifier tube 33. The voltage drop across inductor 45 is applied to thecathode of the other rectifier unit of vacuum tube 33. The voltage dropacross the quartz crystal 44 is applied to the cathode and anode notpreviously mentioned.

Referring to Fig. 3, the voltage drop across the capacitor 43 is shownby the line A, that across the inductor 45 by the line B and thatacrossthe quartz crystal by the line C.

Referring to Fig. 4, the relative phase and approximate magnitude ofthese'voltage drops are shown. The voltage obtained from the oscillatingtank circuit is assumed to be vertical and several times larger than anyvector shown. The voltage drop across the capacitor 43 is indicated asa, and that across inductor 45 by vector 6. These are for a frequencynear the desired frequency of oscillation but, as may be readily seenfrom later discussion, have little effect on the stability of thesystem. Vector 0 shows the voltage drop across the quartz crystal whenthe ap plied frequency is near the resonant point of the crystal. Vectora shows this voltage drop for a frequency above the resonant frequencyof the quartz crystal 44 and vector e for a frequency below the resonantfrequency. 1

The resonant frequency of the oscillator tank circuit is chosen slightlybelow the desiredfrequency (resonant frequency of the quartzcrystal) andthe inductor 41 so chosen that, when it is shunted across'a portion ofthe oscillator tank circuit, the frequency will be above the desiredfrequency. Furthermore, the coupling be tween the inductor 41 and theoscillator tank circuit by means of the vacuum tube 32 is of the amountnecessary to bring frequency of oscillation to the desiredfrequencywhentheva'c uum tube 32 is at a point-on its operatingchai'acteristic near the center of its: normal range of operation.

If the oscillation frequency is above the resonant frequency'of thequartz crystal 44, the difference between vectors 2) and (I will be muchsmaller than the difference between vectors a and (1. Therefore, morecurrent will be rectified by the rectifier unit shown at the right thanthe unit at the left, and the voltage across capacitor 46 will becomemore negative with respect to ground. This reduces. the grid voltage ofthe reactance control tube 32 thereby reducing the coupling betweeninductor 41 and the oscillator tank circuit which reduces the frequencyof oscillation.

If the oscillation frequency is below the resonant frequency of thequartz crystal, the difference between vectors a and e will be small incomparison with the difference between 'vectom 2) and e. Therefore, morecurrent will be rectified by the rectifierunit at the left than by theunit at the right, and the capacitor 46 will be charged positively. Thisincreases the grid voltage of the reactance control tube 32 therebyincreasing the coupling between inductor 41 and the oscillator tankcircuit, consequently increasing the frequency of oscillation. Radiofrequency choke coil 48 serves to. conduct rectified current around thecapacitor 43. This is necessary to preserve symmetry and balance of thecircuits associated with the two rectifiers. Thus it may be seen thatthe frequency of oscillation will be automatically corrected to coincidewith the resonant frequency of the quartz crystal.

The design of the circuit described above may be made so that theoscillator may be frequency modulated by any of the well-known methodsand the average frequency automatically corrected to coincide with theresonant frequency of the quartz crystal through the rectifying actionof the rectifier units and integration of the rectified currents in thecapacitor 46.

It is obvious to one skilled in the art, that the vacuum tubes 3! and32, which are shown as separate triodes, may be two triodesin a commonenvelope or may be pentodes or combinations of a triode and a pentodeand may be in a common or in separate envelopes. Furthermore, the systemdescribed may be used with the diode rectifiers in separate envelopes.

What I claim is:

l. A system for stabilizing the frequency of an oscillator generatorhaving a variable frequencydetermining element and, a crystal stabilizertherefor, which comprises means to develop in separate paths twovoltages which lag and lead respectively the phase of generator voltageby predetermined amounts when the generator frequency is substantiallythe same as the resonant crystal frequency, means to excite the crystalin phase with the generator voltage, means to compare the relative.phases between the .crystal voltage and said two voltages, and meansautomatically responsive to -said comparison to maintain the generatorfrequency at said resonant crystal frequency.

2. A system for stabilizing the frequency of an oscillator generatorhaving a variable frequencydetermining element and a crystal stabilizertherefor, whichcomprises means to excite the crystal substantially inphase with the generator Voltage, means to develop in two separate pathsunder control of the generator voltage two voltages which aresubstantially equal in magnitude but which respectively lead and lag thegenerator voltage bypredetermined amounts. when thegenerator frequencyis substantially the same as the resonant crystal frequency, meansto.develop a frequency correction voltage which is a function of the phasedifferences of said two voltages with respect to the crystal voltage,and means to applysaid correction voltage'to control the effectivenessof said frequency determining element.

3. A system according to claim 2 in which the means to develop saidcorrection voltage includes a pair of electron streams betweencorresponding pairs of cathodes and anodes with a pair of grids in eachstream one grid ofeach pair being excited in phase with the crystalexcitation the other grids of each pair being excited respectively bysaid two voltages.

4. A system according to claim 2 in which the means to develop saidcorrection voltage includes a pair of multi-grid electron tubes eachhaving a first grid and a second grid the first grid of each tube beingexcited in phase with the excitation of the crystal and the second gridofeach tube being excited respectively by one of said two voltages.

5. A system according to claim 2 in which the means to develop saidcorrection Voltage includes a pair of multi-grid electron tubes of theshield grid type the control grids of the tubes being excited in phasewith the excitation of the crystal and the shield grids being eachexcited respectively by one of said two voltages.

I 6. A system according to claim 2 in which the meansto develop saidcorrection voltage includes a pair of pentode tubes with theirsuppressor grids connected to their cathodes, with the control gridsexcited in phase with the excitation of the crystal and the shield gridseach excited respectively by one of said two voltages.

7. A system according to claim 2 in which the means to. develop saidcorrection voltage includes a double rectifier one section of which iscontrolled by the voltage diiference between the crystal voltage and oneof said two voltages, and the other section of which is controlled bythe voltage difference between the crystal voltage and the other of saidtwo voltages.

8. A system for stabilizing the frequency of an electron tubeoscillation generator having a frequency-determining element connectedto a tank circuit and whose effectiveness is controlled by a variableimpedance grid-controlled electron tube, which comprises three voltagepaths the first being predominantly capacitive, the second beingpredominantly inductive, and the third including a frequency stabilizingcrystal, means to apply to said paths a voltage of the generatorfrequency and derived from the tank circuit, means to adjust the phaseof each path so that when the generator frequency is substantially thesame as the resonant crystal frequency the phase differences between thecrystal path and the other two paths is ineffective to vary theimpedance of said variable impedance tube, and means effective when thegenerator frequency departs from the r resonant crystal frequency andcontrolled by the relative phase differences between said two paths andthe crystal path to apply a voltage to the control grid of said variableimpedance tube to restore the generator frequency to that of theresonant crystal frequency.

9. A system according to claim 8 in which the crystal path is excited inphase with the generator voltage.

10. A system according to claim 8 in which the voltages developed insaid two other paths are substantially the same magnitude when thegenerator frequency is the same as the resonant crystal frequency.

' 1 1. A system for stabilizing the frequency of an electron tubeoscillation generator, comprising a pair of grid-controlled tubes, threepaths excited from the tank circuit of the generator, the first pathincluding a crystal stabilizer, the second path having a leadingcharacteristic with relation to the crystal path, and the third pathhaving a lagging characteristic with relation to the crystal path, meansto connect said three paths to both said grid-controlled tubes so thatthe plate current of one tube'is a function of the phase difierencebetween the first path and the crystal, and the plate current of theother tube is a function of the phase difierence between the third pathand the crystal path, and means to develop a frequency correctionvoltage which is a resultant of the plate currents of said pair oftubes.

12. A system for stabilizing the frequency of an electron tubeoscillator generator, comprising three paths connected to the tankcircuit of the generator, one of said paths having a resonant crystalvoltage-frequency characteristic curve, the second of said paths havinga falling characteristic curve with the voltage drop across the pathfalling as the frequency is increased, the third of said paths having arising characteristic curve with the voltage drop across the path risingas the frequency is increased, means to adjust the impedances of eachpath so that when the resonant crystal frequency is substantially thesame as the generator frequency the point of insection of thecharacteristic curves of the second and third paths is locatedsubstantially at the resonant frequency of the crystal, and means todevelop a frequency correction voltage for the generator which is afunction of the resultant of the differences between the voltages insaid second and third paths with respect to the voltage in the crystalpath.

13. A system according to claim 12 in which the last-mentioned meanscomprises a rectifier having two sections, the said first path beingconnected between the cathode of one rectifier section and ground, thesecond path being connected between the anode of said one rectifiersection and ground, the third path being connected between the cathodeof the other rectifier section and ground, said crystal path being alsoconnected to the anode of said other rectifier section.

14. A system for stabilizing the frequency of an electron tubeoscillator generator, comprising a rectifier having two sections, a.network having a falling voltage-frequency characteristic and connectedto the anode of one rectifier section. another network having a risingvoltage-frequency characteristic and connected to the cathode of theother rectifier section, a stabilizing crystal connected to the cathodeof the first section and to the anode of the second section, agrid-controlled variable impedance tube, and means to apply to the gridof said tube a voltage which is the resultant of the currents in saidtwo rectifier sections, and means to connect said gridcontrolled tube tothe tank circuit of the generator so that variations in impedance ofsaid tube result in variations of the generator frequency.

15. A system according to claim 14 in which means are provided to adjustthe impedance in each of said paths so that said resultant is of a fixedpredetermined magnitude when the generator frequency is substantiallythe same as the resonant crystal frequency.

16. A frequency stabilizing system for an electron tube oscillatorgenerator having a tuned tank circuit controllable by a reactanceconnected to a grid-controlled variable impedance tube, which comprisesa pair of grid-controlled tubes for selectively controlling themagnitude of the plate current of said impedance tube, a triple branchnetwork for selectively controlling the plate current of said pair oftubes one branch comprising an inductive reactance connected to animpedance regulating grid of one of said pair of tubes, a second branchcomprising a capacitive reactance connected to an impedance regulatinggrid of the other of said pair of tubes, the third branch including acrystal reactance connected to the control grids of both said pairof-tubes, said three branches being excited from said tank circuit.GEORGE T. ROYDEN.

